Xenopsin-related peptides (Non-Patent Documents 1 and 2) can act as neurotransmitters, neuromodulators, and local hormones. In recent years, attention has particularly focused on their function as local hormones.
Xenopsin-related peptides acting as a local hormone are important in that they have gastric control functions (for example, a function that controls mammalian food intake), and are considered effective for treatment of obesity.
It is already known that such xenopsin-related peptides are present in a peptide mixture comprising xenopsin and a variety of xenopsin-related peptides having the same or similar amino acid sequences from the C-terminal and contained in extract from a mammalian stomach, etc. (for example, extract from the stomach, liver, and brain of mouse (Non-Patent Document 3); extract from avian tissue (Non-Patent Document 4); and extract from dogs (Non-Patent Document 5)).
Therefore, an easy and accurate quantification of a specific xenopsin-related peptide in a peptide mixture is very important.
Known methods include a method comprising purifying a peptide mixture using various chromatography methods, and examining the purified peptide mixture using an immunoassay. The immunoassay has high sensitivity for a peptide with a widely different amino acid sequence.
However, xenopsin and a variety of xenopsin-related peptides in the extract have the same or similar amino acid sequences from the C-terminal; therefore, it is difficult to distinguish a specific xenopsin-related peptide from other peptide compounds to accurately quantify the specific peptide in immunoassay.    Non-Patent Document 1:    Sures, I.; Crippa, M. Proc. Natl. Acad. Sci. 1984, 81, p 380    Non-Patent Document 2:    Bevins, C. L.; Zasloff, M. Annu. Rev. Biochem. 1990, 59, p 395    Non-Patent Document 3:    Carraway, R. E.; Mitra, S. P.; Muraki, K. Regul. Pept. 1990, 29, p 229-239    Non-Patent Document 4:    Carraway, R. E.; Cochrane, D. E.; Mitra, S. P. Regul. Pept. 1988, 22, p 303-314    Non-Patent Document 5:    Carraway, R. E.; Mitra, S. P. Peptides. 1990, 11, p 747-752